They didn't have a bad season, Dortmund was racing with Bayern Munich until last week and that's good for them. They had the chance to win the title until the last minute and maybe in the next years, they won't get this chance anymore. That's why some people think they had a bad performance because they expect to see Dortmund winning the last season.
However, by selling Jude Bellingham they made good money in this season.
We witnessed and observed all that happened the previous season, including top-tier rivalry for the league title from both outstanding clubs in the league, Borrusia Dortmund and Bayern Munich; it was not a strenuous struggle, but at the end of the season, the best team wins. We have plenty of time to map out the statistics of these teams; Dortmund put in a great deal of work and still obtained bad results, losing the title on the final day, which was a huge disappointment. Bayern Munich is the most competitive club in the Bundesliga, followed by Borussia Dortmund, who are just no match for them. On a second thought, I think Dortmund is all about doing businesses that would result in gigantic profits for them.